Question

The boiling points of HF, HCl, HBr, and HI follow the order:
a) HF > HCl > HBr > HI
b) HF > HI > HBr > HCl
c)HI > HBr > HCl > HF
d) HCl > HF > HBr > HI

Answer 1

Hint: The boiling point of a given molecule depends upon the intermolecular interactions. Higher the strength of these interactions, higher is the value of boiling point of molecules. So, we can determine the order followed by the given molecules.

Complete answer: First, let us discuss the given molecules i.e. hydrogen halides. F, Cl, Br, and I belong to the group of halogens.
>Talking about the boiling point, it depends upon the hydrogen bonding present between the molecules.
>Now, if we consider the above molecules, i.e. HI, HBr, and HCl. In these molecules, the dominating interactions would be van der Waals forces. As among these three halides, HI has the larger size or molecular mass.
>Then we know, larger the size, or molecular mass of hydrogen halides, greater will be the van der Waals forces, higher the boiling point. The presence of Hydrogen bonding is negligible in these halides.
>If we talk about the HF, it has strong hydrogen bonding, thus has the highest boiling point.
>Therefore, HF exists in the liquid state, and HI, HBr, and HCl exists in the gaseous state.
>We can conclude, the order of boiling point followed is HF > HI > HBr > HCl.
>Hence, the correct option is (B).

Note: Don’t get confused between the boiling points of HF, and HI. The confusion can occur why HF has a higher boiling point than the HI. The reason is the presence of strong hydrogen bonding in the HF, as hydrogen intermolecular interactions are greater than the van der Waals forces. So, HF has a higher boiling point than the HI.